Metabolic processes have long been proposed for anabolic and catabolic bioconversions. Microorganisms of various types have been proposed for these bioconversions and include bacteria and archaea, both of which are prokaryotes; fungi; and algae. Metabolic processes are used by nature, and some have been adapted to use by man for millennia for anabolic and catabolic bioconversions ranging from culturing yogurt and fermentation of sugars to produce alcohol to treatment of water to remove contaminants. Metabolic processes offer the potential for low energy consumption, high efficiency bioconversions in relatively inexpensive processing equipment and thus may be and are often viable alternatives to chemical synthesis and degradation methods. Often anabolic processes can use raw materials that are preferred from a renewable or environmental standpoint but are not desirable for chemical synthesis, e.g., the conversion of carbon dioxide to biofuels and other bioproducts. Catabolic bioconversions can degrade substrates and have long been used for waste water treatment. Considerable interests exist in improving metabolic processes for industrial use and expanding the variety of metabolic process alternatives to chemical syntheses and degradations.
Virtually all microorganisms require the presence of water in liquid or vaporous form. Consequently, metabolic processes that have been proposed by prior workers involve the contact of an aqueous medium with the microorganisms. The aqueous medium may be a continuous phase in which the microorganisms exist or may be an aqueous phase wetting the microorganisms or support on which the microorganisms reside.
The use of an aqueous medium can pose challenges. For instance, one or more substrates or one or more metabolic products may have little, if any solubility in water, thereby presenting mass transfer challenges. Another challenge, is the separation of the sought bioproduct from an aqueous medium. Distillation is an often used process, which can be energy intensive especially where the boiling points of the sought bioproduct and water are proximate. With some oxygenated organic compounds such as alcohols, azeotropes can form thereby presenting additional problems in the separation of the sought bioproduct from water.
Some disclosed bioprocesses inherently or by choice generate relatively dilute broths of the sought bioproduct in water. Woods, et al., in U.S. Pat. No. 7,682,821 B2, disclose a photobioreactor system in which solar energy is used to provide a concentrated ethanol product that can be more economically distilled. The patentees state that photobioreactor systems experience various limitations for use in massive scale industrial production of low-cost biofuels. One such limitation is stated to be the evaporation of ethanol from the aqueous medium. The photobioreactor comprises a chamber having a translucent or clear region to allow in sunlight to contact an aqueous fermentation medium in a lower part of the chamber wherein ethanol and water vapors can condense on the upper part of the chamber. The condensate has an enhanced ethanol concentration thus enabling more economic recovery of an anhydrous ethanol product.
In other instances, for example, fermentation processes to make butanols, including, but not limited to n-butanol and isobutanol, the butanol in the aqueous medium must be maintained relatively dilute since the butanol is toxic or inhibitory toward the microorganisms. In general, the concentration of butanol is less than about 3 volume percent in the aqueous medium. Due to the amount of water required to be removed, distillation is not the preferred recovery process. Workers have sought to modify microorganisms to increase tolerance. See, for instance, U.S. Patent Application Publication 2010/0105103 A1. Workers have also proposed extraction of butanol from the aqueous medium to maintain the butanol concentration below that which adversely affects the microorganisms. Erdner-Tindall, et al., in U.S. Patent Application Publication No. 2010/0143993 A1 have proposed processes for producing alcohols using an ionic liquid as a solvent to separate an alcohol product from a fermentation broth. They state that ionic liquids will generally have no measurable vapor pressure, have a high solubility for alcohol product, and are immiscible with the aqueous fermentation broth. They further state that the ionic liquid has little to no toxicity to the microorganism.
A prior disclosure of extraction of some bioproducts from aqueous media was made by Tedder in U.S. Pat. No. 4,517,298. The patentee discloses a process for producing simple aliphatic alcohols, most particularly ethanol, using an organic solvent containing an extractant for contact with aqueous fermentation medium withdrawn from fermentation unit. An alcohol-solvent extract phase and an aqueous phase are formed, and the alcohol is separated from the alcohol-solvent phase, e.g., by evaporation or distillation. The patentee contemplates returning the aqueous phase to the fermentation unit. The solvent system disclosed comprises a hydrophobic solvent, such as aliphatic hydrocarbon, and extractant.
Shirazi, et al., in the above-mentioned U.S. patent application Ser. No. 13/918,868 disclose biocatalysts having a high tolerance to the presence of toxins. These biocatalysts comprise                i. a solid structure of hydrated hydrophilic polymer defining an interior structure having a plurality of interconnected major cavities having a smallest dimension of between about 5 to about 100 microns and an HEV of at least about 1000 and        ii. a population of microorganisms capable of converting sugars to at least one organic product substantially irreversibly retained in the interior of the solid structure, said population of microorganisms being in a concentration of at least about 60 grams per liter based upon the volume defined by the exterior of the solid structure when fully hydrated wherein the microorganisms maintain their population substantially stable.        
The microorganisms are believed to undergo phenotypic alterations enabling, inter alia, enhanced tolerance. The disclosed biocatalysts are particularly attractive for continuous processes for the bioconversion of fermentable sugars to ethanol as the biocatalyst is substantially devoid of solids generation, and, being a solid, enables separation of the biocatalyst from the fermentation broth. Additionally, the phenotypic alterations reduce the requirement of the microorganism for sugars for metabolic sustenance thereby enabling the bioconversion of as much as about 99 percent of the fermentable sugars to bioproducts. Moreover, the biocatalyst has a long lifetime and competition with undesired microorganism is substantially eliminated. For ease of reference, these biocatalysts are herein referred to as ME biocatalysts.
Improved processes are sought for bioconverting substrate to bioproduct where the microorganisms need to be retained in an aqueous medium.